Open Access. Powered by Scholars. Published by Universities.®

Physical Sciences and Mathematics Commons™

Open Access. Powered by Scholars. Published by Universities.®

2008

Dartmouth College

Articles 1 - 30 of 84

Full-Text Articles in Physical Sciences and Mathematics

Topological Structures In The Equities Market Network, Gregory Leibon, Scott Pauls, Daniel Rockmore, Robert Savell Dec 2008

Topological Structures In The Equities Market Network, Gregory Leibon, Scott Pauls, Daniel Rockmore, Robert Savell

Dartmouth Scholarship

We present a new method for articulating scale-dependent topological descriptions of the network structure inherent in many complex systems. The technique is based on “partition decoupled null models,” a new class of null models that incorporate the interaction of clustered partitions into a random model and generalize the Gaussian ensemble. As an application, we analyze a correlation matrix derived from 4 years of close prices of equities in the New York Stock Exchange (NYSE) and National Association of Securities Dealers Automated Quotation (NASDAQ). In this example, we expose (i) a natural structure composed of 2 interacting partitions of …

Go to article

The Acs Survey Of Galactic Globular Clusters. Vi. Ngc 6366: A Heavily Stripped Galactic Globular Cluster, Nathaniel E. Q. Paust, Antonio Aparicio, Giampaolo Piotto, I. Neill Reid, Jay Anderson, Ata Sarajedini, Luigi R. Bedin, Brian Chaboyer Dec 2008

The Acs Survey Of Galactic Globular Clusters. Vi. Ngc 6366: A Heavily Stripped Galactic Globular Cluster, Nathaniel E. Q. Paust, Antonio Aparicio, Giampaolo Piotto, I. Neill Reid, Jay Anderson, Ata Sarajedini, Luigi R. Bedin, Brian Chaboyer

Dartmouth Scholarship

We have used observations obtained as part of the Hubble Space Telescope/ACS Survey of Galactic globular clusters (GCs) to construct a color-magnitude diagram for the bulge cluster, NGC 6366. The luminosity function derived from those data extends to M F606W ~ 9, or masses of ~0.3 M ☉. Unlike most GCs, the mass function peaks near the main-sequence turnoff with significantly fewer low-mass stars even after correction for completeness and mass segregation. Using a multimass King model, we extrapolate the global cluster behavior and find the global mass function to be poorly matched by a power law, with …

Go to article

Direct Distance Measurement To The Dusty White Dwarf Gd 362, Mukremin Kilic, John R. Thorstensen, D. Koester Dec 2008

Direct Distance Measurement To The Dusty White Dwarf Gd 362, Mukremin Kilic, John R. Thorstensen, D. Koester

Dartmouth Scholarship

We present trigonometric parallax observations of GD 362 obtained over seven epochs using the MDM 2.4m Hiltner Telescope. The existence of a dust disk around this possibly massive white dwarf makes it an interesting target for parallax observations. The measured parallax for GD 362 places it at a distance of 50.6 pc, which implies that its radius and mass are ~ 0.0106 Rsun and 0.71 Msun, respectively. GD 362 is not as massive as initially thought (1.2Msun). Our results are entirely consistent with the distance and mass estimates (52.2 pc and 0.73 Msun) by Zuckerman et al., who demonstrated that …

Go to article

Nymble: Blocking Misbehaving Users In Anonymizing Networks, Patrick P. Tsang, Apu Kapadia, Cory Cornelius, Sean W. Smith Dec 2008

Nymble: Blocking Misbehaving Users In Anonymizing Networks, Patrick P. Tsang, Apu Kapadia, Cory Cornelius, Sean W. Smith

Computer Science Technical Reports

Anonymizing networks such as Tor allow users to access Internet services privately by using a series of routers to hide the client's IP address from the server. The success of such networks, however, has been limited by users employing this anonymity for abusive purposes such as defacing popular websites. Website administrators routinely rely on IP-address blocking for disabling access to misbehaving users, but blocking IP addresses is not practical if the abuser routes through an anonymizing network. As a result, administrators block \emph{all} known exit nodes of anonymizing networks, denying anonymous access to misbehaving and behaving users alike. To address …

Go to article

Statistics Of Auroral Langmuir Waves, M. Samara, J. Labelle, I. H. Cairns Dec 2008

Statistics Of Auroral Langmuir Waves, M. Samara, J. Labelle, I. H. Cairns

Dartmouth Scholarship

The Physics of Auroral Zone Electrons II (PHAZE II) sounding rocket was launched in February 1997 into active pre-midnight aurora. The resulting high frequency wave data are dominated by Langmuir waves. Consistent with many previous observations the Langmuir waves are sporadic, occurring in bursts lasting up to a few hundred ms. We compute statistics of the electric field amplitudes of these Langmuir waves, with two results. First, the shape of the distribution of running averages of the electric field amplitudes remains approximately stationary for a large range of widths of running average less than ~0.3 ms and for a large …

Go to article

Functional Monitoring Without Monotonicity, Chrisil Arackaparambil, Joshua Brody, Amit Chakrabarti Dec 2008

Functional Monitoring Without Monotonicity, Chrisil Arackaparambil, Joshua Brody, Amit Chakrabarti

Computer Science Technical Reports

The notion of distributed functional monitoring was recently introduced by Cormode, Muthukrishnan and Yi to initiate a formal study of the communication cost of certain fundamental problems arising in distributed systems, especially sensor networks. In this model, each of k sites reads a stream of tokens and is in communication with a central coordinator, who wishes to continuously monitor some function f of \sigma, the union of the k streams. The goal is to minimize the number of bits communicated by a protocol that correctly monitors f(\sigma), to within some small error. As in previous work, we focus on a …

Go to article

Digital Image Ballistics From Jpeg Quantization: A Followup Study, Hany Farid Dec 2008

Digital Image Ballistics From Jpeg Quantization: A Followup Study, Hany Farid

Computer Science Technical Reports

The lossy JPEG compression scheme employs a quantization table that controls the amount of compression achieved. Because different cameras typically employ different tables, a comparison of an image's quantization scheme to a database of known cameras affords a simple technique for confirming or denying an image's source. This report describes the analysis of quantization tables extracted from 1,000,000 images downloaded from Flickr.com.

Go to article

Anomalies In Electrostatic Calibrations For The Measurement Of The Casimir Force In A Sphere-Plane Geometry, W. J. Kim, M. Brown-Hayes, D. A.R. Dalvit, J. H. Brownell, R. Onofrio Dec 2008

Anomalies In Electrostatic Calibrations For The Measurement Of The Casimir Force In A Sphere-Plane Geometry, W. J. Kim, M. Brown-Hayes, D. A.R. Dalvit, J. H. Brownell, R. Onofrio

Dartmouth Scholarship

We have performed precision electrostatic calibrations in the sphere-plane geometry, and observed anomalous behavior. Namely, the scaling exponent of the electrostatic signal with distance was found to be smaller than expected on the basis of the pure Coulombian contribution, and the residual potential found to be distance dependent. We argue that these findings affect the accuracy of the electrostatic calibrations and invite reanalysis of previous determinations of the Casimir force.

Go to article

Group-Aware Stream Filtering For Bandwidth-Efficient Data Dissemination, Ming Li, David Kotz Dec 2008

Group-Aware Stream Filtering For Bandwidth-Efficient Data Dissemination, Ming Li, David Kotz

Dartmouth Scholarship

In this paper we are concerned with disseminating high-volume data streams to many simultaneous applications over a low-bandwidth wireless mesh network. For bandwidth efficiency, we propose a group-aware stream filtering approach, used in conjunction with multicasting, that exploits two overlooked, yet important, properties of these applications: 1) many applications can tolerate some degree of “slack” in their data quality requirements, and 2) there may exist multiple subsets of the source data satisfying the quality needs of an application. We can thus choose the “best alternative” subset for each application to maximize the data overlap within the group to best benefit …

Go to article

Metallicity Analysis Of Macho Galactic Bulge Rr0 Lyrae Stars From Their Light Curves, Andrea Kunder, Brian Chaboyer Dec 2008

Metallicity Analysis Of Macho Galactic Bulge Rr0 Lyrae Stars From Their Light Curves, Andrea Kunder, Brian Chaboyer

Dartmouth Scholarship

We present metallicities of 2690 RR0 Lyrae stars observed toward the MACHO Survey fields in the Galactic bulge. These [Fe/H] values are based upon an empirically-calibrated relationship that uses the Fourier coefficients of the light curve and are accurate to ±0.2 dex. The majority of the RR0 Lyrae stars in our sample are located in the Galactic bulge, but 255 RR0 stars are associated with the Sagittarius (Sgr) dwarf galaxy. We find that the RR0 Lyrae stars that belong to the Galactic bulge have average metallicities [Fe/H] = -1.25, with a broad metallicity range from [Fe/H] = -2.26 to -0.15. …

Go to article

Parameters Of Pseudorandom Quantum Circuits, Yaakov S. Weinstein, Winton G. Brown, Lorenza Viola Nov 2008

Parameters Of Pseudorandom Quantum Circuits, Yaakov S. Weinstein, Winton G. Brown, Lorenza Viola

Dartmouth Scholarship

Pseudorandom circuits generate quantum states and unitary operators which are approximately distributed according to the unitarily invariant Haar measure. We explore how several design parameters affect the efficiency of pseudorandom circuits, with the goal of identifying relevant tradeoffs and optimizing convergence. The parameters we explore include the choice of single- and two-qubit gates, the topology of the underlying physical qubit architecture, the probabilistic application of two-qubit gates, as well as circuit size, initialization, and the effect of control constraints. Building on the equivalence between pseudorandom circuits and approximate t-designs, a Markov matrix approach is employed to analyze asymptotic convergence properties …

Go to article

Quantum Nondemolition Measurement Of Discrete Fock States Of A Nanomechanical Resonator, E. Buks, E. Segev, S. Zaitsev, B. Abdo, M. P. Blencowe Nov 2008

Quantum Nondemolition Measurement Of Discrete Fock States Of A Nanomechanical Resonator, E. Buks, E. Segev, S. Zaitsev, B. Abdo, M. P. Blencowe

Dartmouth Scholarship

We study theoretically a radio frequency superconducting interference device integrated with a nanomechanical resonator and an LC resonator. By applying adiabatic and rotating-wave approximations, we obtain an effective Hamiltonian that governs the dynamics of the mechanical and LC resonators. Nonlinear terms in this Hamiltonian can be exploited for performing a quantum nondemolition measurement of Fock states of the nanomechanical resonator. We address the feasibility of experimental implementation and show that the nonlinear coupling can be made sufficiently strong to allow the detection of discrete mechanical Fock states.

Go to article

Pfisr Nightside Observations Of Naturally Enhanced Ion Acoustic Lines, And Their Relation To Boundary Auroral Features, R. G. Michell, K. A. Lynch, C. J. Heinselman, H. C. Stenbaek-Nielsen Nov 2008

Pfisr Nightside Observations Of Naturally Enhanced Ion Acoustic Lines, And Their Relation To Boundary Auroral Features, R. G. Michell, K. A. Lynch, C. J. Heinselman, H. C. Stenbaek-Nielsen

Dartmouth Scholarship

We present results from a coordinated camera and radar study of the auroral ionosphere conducted during March of 2006 from Poker Flat, Alaska. The campaign was conducted to coincide with engineering tests of the first quarter installation of the Poker Flat Incoherent Scatter Radar (PFISR). On 31 March 2006, a moderately intense auroral arc, (~10 kR at 557.7 nm), was located in the local magnetic zenith at Poker Flat. During this event the radar observed 7 distinct periods of abnormally large backscattered power from the F-region. These were only observed in the field-aligned radar beam, and radar spectra from these …

Go to article

Toward Evaluating Lighting Design Interface Paradigms For Novice Users, William Brandon Kerr, Fabio Pellacini Nov 2008

Toward Evaluating Lighting Design Interface Paradigms For Novice Users, William Brandon Kerr, Fabio Pellacini

Computer Science Technical Reports

Lighting design is a complex and fundamental task in computer cinematography, involving adjustment of light parameters to define final scene appearance. Many lighting interfaces have been proposed to improve lighting design work flow. These paradigms exist in three paradigm categories: direct light parameter manipulation, indirect light feature manipulation (e.g., shadow dragging), and goal-based optimization of light through painting. To this date, no formal evaluation of the relative effectiveness of these methods has been performed. In this paper, we present a first step toward evaluating the three paradigms in the form of a user study with novice users. We focus our …

Go to article

A Catalog Of Outer Ejecta Knots In The Cassiopeia A Supernova Remnant, Molly C. Hammell, Robert A. Fesen Nov 2008

A Catalog Of Outer Ejecta Knots In The Cassiopeia A Supernova Remnant, Molly C. Hammell, Robert A. Fesen

Dartmouth Scholarship

Hubble Space Telescope images of the core-collapse supernova remnant Cassiopeia A are used to identify high-velocity knots of ejecta located outside the remnant's main emission shell of expanding debris. These ejecta fragments are found near or ahead of the remnant's forward shock front and mostly lie from 120'' to 300'' in radial distance from the remnant's center of expansion. Filter flux ratios when correlated with published spectra show that these knots can be divided into three emission classes: (1) knots dominated by [N II] λλ6548, 6583 emissions, (2) knots dominated by [O II] λλ7319, 7330 emissions, and (3) …

Go to article

Parallax And Distance Estimates For Twelve Cataclysmic Variable Stars, John R. Thorstensen, Sébastien Lépine, Michael Shara Oct 2008

Parallax And Distance Estimates For Twelve Cataclysmic Variable Stars, John R. Thorstensen, Sébastien Lépine, Michael Shara

Dartmouth Scholarship

We report parallax and distance estimates for 12 more cataclysmic binaries and related objects observed with the 2.4 m Hiltner telescope at MDM Observatory. The final parallax accuracy is typically ~1 mas. Notable results include distances for V396 Hya (CE 315), a helium double degenerate with a relatively long orbital period, and for MQ Dra (SDSSJ155331+551615), a magnetic system with a very low accretion rate. We find that the Z Cam star KT Persei is physically paired with a K main-sequence star lying 15 arcsec away. Several of the targets have distance estimates in the literature that are based on …

Go to article

Blac: Revoking Repeatedly Misbehaving Anonymous Users Without Relying On Ttps, Patrick P. Tsang, Man Ho Au, Apu Kapadia, Sean W. Smith Oct 2008

Blac: Revoking Repeatedly Misbehaving Anonymous Users Without Relying On Ttps, Patrick P. Tsang, Man Ho Au, Apu Kapadia, Sean W. Smith

Computer Science Technical Reports

Several credential systems have been proposed in which users can authenticate to service providers anonymously. Since anonymity can give users the license to misbehave, some variants allow the selective deanonymization (or linking) of misbehaving users upon a complaint to a trusted third party (TTP). The ability of the TTP to revoke a user's privacy at any time, however, is too strong a punishment for misbehavior. To limit the scope of deanonymization, systems have been proposed in which users are deanonymized if they authenticate ``too many times,'' such as ``double spending'' with electronic cash. While useful in some applications, it is …

Go to article

The Changing Usage Of A Mature Campus-Wide Wireless Network, Tristan Henderson, David Kotz, Ilya Abyzov Oct 2008

The Changing Usage Of A Mature Campus-Wide Wireless Network, Tristan Henderson, David Kotz, Ilya Abyzov

Dartmouth Scholarship

Wireless Local Area Networks (WLANs) are now commonplace on many academic and corporate campuses. As "Wi-Fi" technology becomes ubiquitous, it is increasingly important to understand trends in the usage of these networks. This paper analyzes an extensive network trace from a mature 802.11 WLAN, including more than 550 access points and 7000 users over seventeen weeks. We employ several measurement techniques, including syslog messages, telephone records, SNMP polling and tcpdump packet captures. This is the largest WLAN study to date, and the first to look at a mature WLAN. We compare this trace to a trace taken after the network's …

Go to article

Probing The Quantum Coherence Of A Nanomechanical Resonator Using A Superconducting Qubit: I. Echo Scheme, A. D. Armour, M. P. Blencowe Sep 2008

Probing The Quantum Coherence Of A Nanomechanical Resonator Using A Superconducting Qubit: I. Echo Scheme, A. D. Armour, M. P. Blencowe

Dartmouth Scholarship

We propose a scheme in which the quantum coherence of a nanomechanical resonator can be probed using a superconducting qubit. We consider a mechanical resonator coupled capacitively to a Cooper pair box and assume that the superconducting qubit is tuned to the degeneracy point so that its coherence time is maximized and the electro-mechanical coupling can be approximated by a dispersive Hamiltonian. When the qubit is prepared in a superposition of states, this drives the mechanical resonator progressively into a superposition which in turn leads to apparent decoherence of the qubit. Applying a suitable control pulse to the qubit allows …

Go to article

Probing The Quantum Coherence Of A Nanomechanical Resonator Using A Superconducting Qubit: Ii. Implementation, M. P. Blencowe, A. D. Armour Sep 2008

Probing The Quantum Coherence Of A Nanomechanical Resonator Using A Superconducting Qubit: Ii. Implementation, M. P. Blencowe, A. D. Armour

Dartmouth Scholarship

We describe a possible implementation of the nanomechanical quantum superposition generation and detection scheme described in the preceding, companion paper (Armour A D and Blencowe M P 2008 New. J. Phys. 10 095004). The implementation is based on the circuit quantum electrodynamics (QED) set-up, with the addition of a mechanical degree of freedom formed out of a suspended, doubly-clamped segment of the superconducting loop of a dc SQUID located directly opposite the centre conductor of a coplanar waveguide (CPW). The relative merits of two SQUID based qubit realizations are addressed, in particular a capacitively coupled charge qubit and inductively coupled …

Go to article

The Acs Survey Of Galactic Globular Clusters. Iii. The Double Subgiant Branch Of Ngc 1851, A. P. Milone, L. R. Bedin, G. Piotto, J. Anderson, I. R. King, A. Sarajedini, A. Dotter, B. Chaboyer, A. Marin Franch, S. Majewski, A. Aparicio, M. Hempel, N. E.Q Paust, I. N. Reid, A. Rosenberg, M. Siegel Sep 2008

The Acs Survey Of Galactic Globular Clusters. Iii. The Double Subgiant Branch Of Ngc 1851, A. P. Milone, L. R. Bedin, G. Piotto, J. Anderson, I. R. King, A. Sarajedini, A. Dotter, B. Chaboyer, A. Marin Franch, S. Majewski, A. Aparicio, M. Hempel, N. E.Q Paust, I. N. Reid, A. Rosenberg, M. Siegel

Dartmouth Scholarship

Photometry with the Hubble Space Telescope Advanced Camera for Surveys (HST ACS) reveals that the subgiant branch (SGB) of the globular cluster NGC 1851 splits into two well-defined branches. If the split is due only to an age effect, the two SGBs would imply two star formation episodes separated by ~1 Gyr. We discuss other anomalies in NGC 1851 that could be interpreted in terms of a double stellar population. Finally, we compare the case of NGC 1851 with the other two globulars known to host multiple stellar populations, and show that all three clusters differ in several important …

Go to article

Quantum Analysis Of A Nonlinear Microwave Cavity-Embedded Dc Squid Displacement Detector, P. D. Nation, M. P. Blencowe, E. Buks Sep 2008

Quantum Analysis Of A Nonlinear Microwave Cavity-Embedded Dc Squid Displacement Detector, P. D. Nation, M. P. Blencowe, E. Buks

Dartmouth Scholarship

We carry out a quantum analysis of a dc superconducting quantum interference device (SQUID) mechanical displacement detector, comprising a SQUID with mechanically compliant loop segment, which is embedded in a microwave transmission line resonator. The SQUID is approximated as a nonlinear current-dependent inductance, inducing an external flux tunable nonlinear Duffing self-interaction term in the microwave resonator mode equation. Motion of the compliant SQUID loop segment is transduced inductively through changes in the external flux threading SQUID loop, giving a ponderomotive radiation pressure-type coupling between the microwave and mechanical resonator modes. Expressions are derived for the detector signal response and noise, …

Go to article

Lzfuzz: A Fast Compression-Based Fuzzer For Poorly Documented Protocols, Sergey Bratus, Axel Hansen, Anna Shubina Sep 2008

Lzfuzz: A Fast Compression-Based Fuzzer For Poorly Documented Protocols, Sergey Bratus, Axel Hansen, Anna Shubina

Computer Science Technical Reports

Real-world infrastructure offers many scenarios where protocols (and other details) are not released due to being considered too sensitive or for other reasons. This situation makes it hard to apply fuzzing techniques to test their security and reliability, since their full documentation is only available to their developers, and domain developer expertise does not necessarily intersect with fuzz-testing expertise (nor deployment responsibility). State-of-the-art fuzzing techniques, however, work best when protocol specifications are available. Still, operators whose networks include equipment communicating via proprietary protocols should be able to reap the benefits of fuzz-testing them. In particular, administrators should be able to …

Go to article

The Evolution Of Late‐Time Optical Emission From Sn 1986j, Dan Milisavljevic, Robert A. Fesen, Bruno Leibundgut, Robert P. Kirshner Sep 2008

The Evolution Of Late‐Time Optical Emission From Sn 1986j, Dan Milisavljevic, Robert A. Fesen, Bruno Leibundgut, Robert P. Kirshner

Dartmouth Scholarship

We present late-time optical images and spectra of the Type IIn supernova SN 1986J. HST ACS/WFC images obtained in 2003 February show it to be still relatively bright, with mF606W = 21.4 and mF814W = 20.0 mag. Compared to 1994 December HST WFPC2 images, SN 1986J shows a decline of only <1 mag in brightness over 8 years. Ground-based spectra taken in 1989, 1991, and 2007 show a 50% decline in Hα emission between 1989 and 1991, and an order of magnitude drop between 1991 and 2007, along with the disappearance of He I line emissions during the period 1991-2007. The object's [O I] λλ6300, 6364, [O II] λλ7319, 7330 and [O III] λλ4959, 5007 emission lines show two prominent peaks near –1000 and –3500 km s−1, with the more blueshifted component declining significantly in strength between 1991 and 2007. The observed spectral evolution suggests two different origins for SN 1986J's late-time optical emission: dense, shock-heated circumstellar material, which gave rise to the initially bright Hα, He I, and [N II] λ5755 …

Go to article

Detecting Kernel Rootkits, Ashwin Ramaswamy Sep 2008

Detecting Kernel Rootkits, Ashwin Ramaswamy

Computer Science Technical Reports

Kernel rootkits are a special category of malware that are deployed directly in the kernel and hence have unmitigated reign over the functionalities of the kernel itself. We seek to detect such rootkits that are deployed in the real world by first observing how the majority of kernel rootkits operate. To this end, comparable to how rootkits function in the real world, we write our own kernel rootkit that manipulates the network driver, thus giving us control over all packets sent into the network. We then implement a mechanism to thwart the attacks of such rootkits by noticing that a …

Go to article

The Dartmouth Stellar Evolution Database, Aaron Dotter, Brian Chaboyer, Darko Jevremović, Veselin Kostov Sep 2008

The Dartmouth Stellar Evolution Database, Aaron Dotter, Brian Chaboyer, Darko Jevremović, Veselin Kostov

Dartmouth Scholarship

The ever-expanding depth and quality of photometric and spectroscopic observations of stellar populations increase the need for theoretical models in regions of age-composition parameter space that are largely unexplored at present. Stellar evolution models that employ the most advanced physics and cover a wide range of compositions are needed to extract the most information from current observations of both resolved and unresolved stellar populations. The Dartmouth Stellar Evolution Database is a collection of stellar evolution tracks and isochrones that spans a range of [Fe/H] from –2.5 to +0.5, [α/Fe] from –0.2 to +0.8 (for [Fe/H] ≤ 0) or +0.2 (for …

Go to article

Streaming Estimation Of Information-Theoretic Metrics For Anomaly Detection (Extended Abstract), Sergey Bratus, Joshua Brody, David Kotz, Anna Shubina Sep 2008

Streaming Estimation Of Information-Theoretic Metrics For Anomaly Detection (Extended Abstract), Sergey Bratus, Joshua Brody, David Kotz, Anna Shubina

Dartmouth Scholarship

Information-theoretic metrics hold great promise for modeling traffic and detecting anomalies if only they could be computed in an efficient, scalable ways. Recent advances in streaming estimation algorithms give hope that such computations can be made practical. We describe our work in progress that aims to use streaming algorithms on 802.11a/b/g link layer (and above) features and feature pairs to detect anomalies.

Go to article

Globular Clusters In The Outer Galactic Halo: Am-1 And Palomar 14, Aaron Dotter, Ata Sarajedini, Soung-Chul Yang Aug 2008

Globular Clusters In The Outer Galactic Halo: Am-1 And Palomar 14, Aaron Dotter, Ata Sarajedini, Soung-Chul Yang

Dartmouth Scholarship

AM-1, at ~120 kpc, and Palomar 14 (Pal 14), at ~70 kpc, are two of the most distant Galactic globular clusters (GCs) known. We present Hubble Space Telescope Wide Field Planetary Camera 2 photometry of AM-1 and Pal 14 that reveals unprecedented depth and detail in the color-magnitude diagrams of these two clusters. Absolute and relative age measurements confirm that both are younger than the inner halo GC M 3 by 1.5-2 Gyr assuming all three clusters have similar compositions. Thus AM-1 and Pal 14 join Pal 3, Pal 4, and Eridanus as distant GCs with red horizontal branches (HBs) …

Go to article

Advantages Of Randomization In Coherent Quantum Dynamical Control, Lea F. Santos, Lorenza Viola Aug 2008

Advantages Of Randomization In Coherent Quantum Dynamical Control, Lea F. Santos, Lorenza Viola

Dartmouth Scholarship

Control scenarios have been identified where the use of randomized design may substantially improve the performance of dynamical decoupling methods (Santos and Viola 2006 Phys. Rev. Lett. 97 150501). Here, by focusing on the suppression of internal unwanted interactions in closed quantum systems, we review and further elaborate on the advantages of randomization at long evolution times. By way of illustration, special emphasis is devoted to isolated Heisenberg chains of coupled spin-1/2 particles. In particular, for nearest-neighbor interactions, two types of decoupling cycles are contrasted: inefficient averaging, whereby the number of control actions increases exponentially with the system size, and …

Go to article

Fish Distributions And Nutrient Cycling In Streams: Can Fish Create Biogeochemical Hotspots, Peter B. Mcintyre, Alexander S. Flecker, Michael J. Vanni, James M. Hood, Brad W. Taylor, Steven A. Thomas Aug 2008

Fish Distributions And Nutrient Cycling In Streams: Can Fish Create Biogeochemical Hotspots, Peter B. Mcintyre, Alexander S. Flecker, Michael J. Vanni, James M. Hood, Brad W. Taylor, Steven A. Thomas

Dartmouth Scholarship

Rates of biogeochemical processes often vary widely in space and time, and characterizing this variation is critical for understanding ecosystem functioning. In streams, spatial hotspots of nutrient transformations are generally attributed to physical and microbial processes. Here we examine the potential for heterogeneous distributions of fish to generate hotspots of nutrient recycling. We measured nitrogen (N) and phosphorus (P) excretion rates of 47 species of fish in an N-limited Neotropical stream, and we combined these data with population densities in each of 49 stream channel units to estimate unit- and reach-scale nutrient recycling. Species varied widely in rates of N …

Go to article

Next Last